Traditionally, commonly used is an oil cooled screw compressor which is cooled with cooling oil between screw rotors and between the screw rotors and rotor chamber. In a conventional oil cooled screw compressor, if the target gas to be compressed is carbon hydrate series gas, the target gas dissolves into the cooling oil to reduce viscosity of the cooling oil, and then an insufficient lubrication of a bearing can be caused to damage the bearing. Further, if the target gas is corrosive gas, the target gas can damage the bearing in the conventional screw compressor.
Patent literature 1 describes a technique to separate target gas dissolved in cooling oil by reducing pressure of target gas discharged from screw compressor in a depression tank. However, it is not able to significantly reduce pressure, and so the deaeration is not always sufficient in the apparatus in the patent literature 1.